The present invention relates to the preparation of sodium silicate from quartz by reaction with caustic soda in aqueous medium, under pressure.
The existing processes for the industrial production of sodium silicate generally comprise:
(i) a calcination of the mixture of sand and sodium carbonate at over 1100.degree. C.;
(ii) a solubilization of the sodium silicate having thus been obtained, in the autoclave at moderate pressure, i.e., 2 to 5 bars; and
(iii) a filtration.
Such processes are costly, since the calcination consumes fuel such as fuel oil or coal. Furthermore, the calcination installation requires a great deal of maintenance because of the possibility of attack on the refractory materials by the sodium carbonate.
Consequently, various efforts have been made in order to eliminate the calcination of the mixture of sand and sodium carbonate and to carry out directly the solubilization of silica by heating in an autoclave with sodium hydroxide; as described more particularly in Chemical Engineering 1962, Feb. 2, p. 76-78 and U.S. Pat. No. 2,881,049.
In order to control and avoid the corrosion of the equipment, especially of the autoclaves, the solubilization is carried out at moderate pressures; i.e., 3 to 6 bars. However, the low temperatures which result from these pressures make it necessary to select the nature of the sand, to crush it or to have this sand undergo a pretreatment generally consisting of a calcination followed by water hardening.
Such processes thus proved to be just as burdensome as the calcination of the mixture of quartz and sodium carbonate, the latter being the only industrial process which is being used.